TR&D 1 Project summary/abstract The goal of this project is to develop positron emission tomography (PET) tracers for imaging the sphingosine- 1-phosphate receptor 1 (S1P1). Sphingosine 1-phosphate (S1P) is a membrane-derived lysophospholipid which plays critical regulatory roles in inflammatory disease through modulating five S1P receptor subtypes. S1P1 is one of the most abundant receptors in this family. Dysregulation of S1P1 signaling is associated with common inflammation-mediated diseases such as involving the central nervous system (e.g., multiple sclerosis [MS]). The cardiovascular system (e.g., atherosclerosis), inflammatory bowel disease and various cancers. The best characterized roles of S1P1 are maintaining endothelial barrier function under both basal and inflammatory conditions, and regulating immune cell trafficking during inflammatory response. The FDA approved S1P- modulator, FTY720 (fingolimod), has been widely used for treatment of relapsing-remitting multiple sclerosis (RR-MS); FTY720 has high affinity for all S1P subtypes except S1P2. Although the positive results in treating RR-MS with FTY720 illustrate the importance of this pathway in chronic inflammatory disease, the mechanism by which S1P1 mediates pathological changes in different diseases is not well understood. A PET tracer with high affinity and selectivity for S1P1 and suitable radiopharmaceutical properties would provide a unique imaging tool to assess quantitatively S1P1 expression in inflammatory tissues. The availability of such a tool would facilitate the study of the role of S1P1 in human disease and potentially lead to diagnostic and therapeutic paradigms. We radiosynthesized a known S1P1 inhibitor, 11C-TZ3321 (IC50 = 2.13 1.63 nM for S1P1, >1000 nM for S1P2 and S1P3) for evaluation in three animal models of inflammatory disease, preliminary results suggest 11C-TZ3321 can be used to quantify S1P1 in vivo. In addition, we also identified several lead compounds for future 18F-labeling that have high potency (S1P1 IC50<10 nM) and selectivity (>100-fold for S1P1 versus S1P2 and S1P3). Therefore, to achieve the goal of the TR&D1 project, we have proposed two specific aims within five years. Our specific aim 1 is to translate 11C-TZ3321 into initial proof of mechanism studies in RR-MS patients. Our specific aim 2 is to develop an 18F-labeled S1P1 specific PET tracer. To accomplish the specific aims, we will work with our collaborative projects (CPs) to evaluate 11C-TZ3321 in different animal models of inflammatory disease and in patients with inflammatory disease. We will provide precursor/standard compounds and protocols including radiochemistry and image analysis to our service projects (SPs) to assist with generating consistent data through their projects. These efforts will characterize the use of an S1P1 tracer for PET imaging of inflammatory disease and our results will be published to enable the wider PET community to use the S1P1 radiotracer developed by this project.